Career-ending injuries for professional rugby players are an occupational hazard but London Wasps continue to suffer more severely than anyone else. The club has already lost six players to early retirement in the past 14 months and that painful list extends to seven following Richard Birkett's decision to hang up his boots following a neck injury.

The long-serving lock, who contributed to Wasps' four Premiership titles between 2003 and 2008 as well as the Heineken Cup triumph in 2004 during his 14-year senior career, has gone the same way as Dan Ward-Smith, Tom Rees, Steve Thompson, Joe Worsley, John Hart and Nic Berry. It is a grim attrition rate and Damian Hopley, chief executive of the Rugby Players' Association, describes it as "a cause for concern, to put it mildly".

"Given the growing collision nature of our sport, the alarm bells are ringing," Hopley said on Wednesday. "You can't fault the medical provision or the training methods but there are still players having to retire prematurely." The RPA anticipates two players at every Premiership club will be forced out of the game through injury each season, twice as many as two years ago.

The 33-year-old Birkett is the latest whose body has "unfortunately given up on me" after taking a heavy hit during an A League game with Saracens Storm this year. Birkett said: "The timing of my departure from the game is disappointing as I will not be able to be part of the new era at Wasps, which has enormous potential.

"However, I leave the game content I was a part of an amazing era in Wasps' history. Those memories will never leave me and I'm proud of what we achieved."

Gloucester's former All Blacks scrum-half Jimmy Cowan makes his first start for the club against Bordeaux Bègles at Kingsholm in the Amlin Challenge Cup on Thursday night. After their ill-tempered clash with Mont-de-Marsan last week, Gloucester are seeking a second successive win and Cowan is one of eight changes to the starting XV. Billy Twelvetrees starts at fly-half, with James Simpson-Daniel, Akapusi Qera, Ben Morgan and Jim Hamilton also recalled.

Francois Louw, meanwhile, will lead Bath away to Agen in his second club appearance of the season. Louw was required by South Africa during the Rugby Championship but is now focused on improving Bath's fortunes. "We are not sure where we want to be as a team," Louw said. "There is a lot of work to be done."

The New Zealand referee Bryce Lawrence has hung up his whistle, citing the intense criticism he received following last year's World Cup. The 41-year-old, who is to become the NZRFU's high-performance referee reviewer, was the subject of widespread criticism following Australia's quarter-final victory over South Africa.

"It got pretty bad," said Lawrence, who took charge of 25 Tests. "On Facebook they launched a 'get rid of Bryce Lawrence' site and it was pretty nasty. That was the reason for my career change."